1. Field of the Invention
The present invention relates to a high withstand voltage semiconductor device used for high current switching or the like.
2. Background Art
Semiconductor devices for power applications such as IGBT are widely used, for example, for inverter circuits for home appliances such as air-conditioners, refrigerators or washing machines for which efforts for energy-saving are being increasingly made, motor control of rolling stock such as the Shinkansen (bullet train) or subway, control of an inverter/converter of hybrid cars or converter circuits for photovoltaic power generation or wind power generation or the like.
It is important that such semiconductor devices have stable and high withstand voltage characteristics during operation (use). For this reason, a variety of structures for alleviating an electric field in outer regions of semiconductor devices have been proposed so far. A guard ring provided in the outer region of a semiconductor device is a typical structure thereof. As another structure, Japanese Patent Laid-Open No. 61-84830 and Japanese Patent Laid-Open No. 1-123478 disclose a VLD (variation of lateral doping) region formed in the outer region. The VLD region is effective compared to the guard ring or the like in that excellent withstand voltage characteristics can be realized while reducing the area thereof.
Semiconductor devices for power applications are each provided with a device region where a device is formed on a top surface side of a semiconductor substrate and an electric field limiting region that surrounds a perimeter thereof. The range of this device region includes an active region where a main current flows as well as a portion where a control electrode pad or the like is formed when the device is a transistor, for example. When an end of the active region in a cross-sectional view is in contact with a region having a conduction type opposite to that of the active region, a PN junction having a large curvature may be formed. This is because the depth of the active region is normally determined in accordance with operating characteristics of the device, and when the depth of the active region decreases, the curvature of a PN junction increases, making it more likely to produce a problem that an electric field and a current concentrate. Therefore, the end of the active region is covered with a well region which has the same conductive type as that of the active region and which is formed deeper than the active region to prevent formation of any PN junction having a greater curvature.
However, since the well region is formed deeper than the active region, a curved portion is also formed in the portion on an inner circumferential side (active region side) of the well region, resulting in a problem that a PN junction partially having a large curvature is formed depending on a pattern shape. There is another problem that an electric field and a current concentrate on a PN junction having a large curvature.